Method to separate an emulsion in a liquid lens

ABSTRACT

Embodiments generally relate to systems and methods for separating an emulsion in a cavity of a device such as a liquid lens device. In one embodiment, the method comprises at least one of: applying a bias voltage to electrodes in the device, causing at least one of droplet migration, flattening of large droplets, and reduced droplet surface tension; applying an oscillating actuation voltage waveform comprising an actuation frequency to the electrodes, such that fluid pumping and turbulence is created within the device cavity; and applying an oscillating excitation voltage waveform comprising an excitation frequency to the electrodes, such that the varying electric field created by the oscillating voltage causes small droplets of the first liquid to coalesce.

RELATED APPLICATIONS

This application claims priority from U.S. Provisional PatentApplication Ser. No. 62/099,097, entitled “Method to remove emulsion ina liquid lens”, filed on Dec. 31, 2015, which is hereby incorporated byreference as if set forth in full in this application for all purposes.

BACKGROUND

An emulsion is defined as a fine dispersion of minute droplets of afirst liquid in a second liquid in which the first liquid is not solubleor miscible. The emulsion can occur by vigorous mixing or shaking of theliquid mixture. A liquid lens (such as those produced by Optilux ofSanta Barbara, Calif.) is created by enclosing two fluids with anengineered index of refraction difference within a housing. The housingincorporates electrodes which manipulate the shape of the lens based onthe principles of electro-wetting. In the case of a liquid lens, anemulsion of the fluids in undesirable as it will negatively impact theoptical performance of the lens. This can occur if the lens is subjectto shock loads, such as caused by an impact due to dropping the device.

Methods by which an emulsion of two fluids may be “demulsified” orseparated back into its two fluid constituents are known in fields suchas the oil/gas industry. For example, a centrifuge can be used toseparate the two fluids based on differences in their density. Anotherprior art method to separate an emulsion is to use chemicals that affectthe surface tension of the fluids. Yet another method involves usingelectric fields to induce coalescence of the droplets of one fluid.However, there is no known prior art that specifically addresses theproblem of an emulsion in a liquid lens cavity, and in particular anemulsion caused by severe shock loading of a liquid lens deviceincluding such a cavity.

The need therefore exists for methods and systems specifically tailoredto clear or remove an emulsion from the field of view of a liquid lens.Ideally the clearing would occur very quickly (in less than 10 seconds,for example) without the need for deliberate user input or theinvolvement of devices external to the liquid lens system. The clearingcould occur automatically during power up, or may be initiated inresponse to a signal from a sensor.

SUMMARY

The present invention includes a method for separating an emulsioncomprising droplets of a first liquid suspended in a second liquidwithin a cavity of a liquid lens device. The method comprises at leastone of: applying a bias voltage to electrodes in the liquid lens device,causing at least one of droplet migration, flattening of large droplets,and reduced droplet surface tension; applying an oscillating actuationvoltage waveform comprising an actuation frequency to the electrodes,such that fluid pumping and turbulence is created within the liquid lenscavity; and applying an oscillating excitation voltage waveformcomprising an excitation frequency to the electrodes, such that thevarying electric field created by the oscillating voltage causes smalldroplets of the first liquid to coalesce.

In one aspect, the bias voltage applied to the electrodes encouragesmigration of the first liquid towards one side of the liquid lens cavityand migration of the second liquid towards an oppositely situated sideof the liquid lens.

In one aspect, the excitation and oscillating actuation waveforms areapplied, and the actuation frequency is lower than the excitationfrequency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross sectional view of a prior art liquid lensthat may be operated on according to one embodiment.

FIG. 2 is a flowchart of a method for removing an emulsion in a liquidlens according to one embodiment.

FIG. 3 illustrates sequential top-down views of a liquid lens in whichan emulsion, clearly visible in the top left view, is graduallyseparated to provide a clear liquid lens in the bottom right view,according to one embodiment.

FIG. 4A illustrates side view images of the early stages of dropletcoalescence according to one embodiment.

FIG. 4B illustrates side view images showing coalescence of two out ofthree droplets according to one embodiment.

FIG. 5 illustrates excitation and actuation voltage waveforms accordingto one embodiment.

FIG. 6 illustrates a swept sine excitation voltage waveform and a liquidlens response according to one embodiment.

FIG. 7 illustrates four possible combinations of excitation andactuation voltage waveforms according to some embodiments.

FIG. 8 illustrates an excitation voltage waveform including a dwell timeaccording to one embodiment.

DETAILED DESCRIPTION

The manner in which the present invention provides its advantages can bemore easily understood with reference to FIGS. 1 through 8.

FIG. 1 is a schematic cross sectional view of a prior art liquid lens100 that may benefit by being operated on according to embodiments ofthe present invention. Liquid lens cavity 101 contains polar liquid 114and non-polar liquid 116. Liquids 114 and 116 are chosen to benon-miscible and of different refractive indices, but of similarspecific gravity, as required for liquid lens functionality. They may,for example, comprise an aqueous component and an oil componentrespectively. The base and top plates bounding lens cavity 101 aretransparent, at least in their central regions directly underlying andoverlying the cavity, at the wavelength of intended operation of theliquid lens. A layer of insulating material 112 lies on the side wallsof cavity 101 on top of electrodes 106.

FIG. 2 is a flowchart illustrating one embodiment of a method 200 forcarrying out the present invention. The illustrated embodiment showsthree steps, 202, 204, 206, but it should be noted that first, any oneor two of the steps may be omitted, and second, the order in which thesteps are shown is arbitrary. In some embodiments, any two or all threesteps may be carried out simultaneously.

At step 202, a bias voltage is applied to electrodes of the liquid lens.Typically, this will be a simple DC voltage applied across the sameelectrodes 106 used to actuate the liquid lens in its normal, focusingapplications, independent of any shock event. At step 204, anoscillating actuation voltage waveform is applied to liquid lensactuation electrodes 106. At step 206, an excitation voltage waveform isapplied to liquid lens electrodes. Again, this will typically be thesame electrodes 106 used to actuate the liquid lens in its normal,focusing applications.

Consider some of the cases of interest to the present invention, whereshock has caused an emulsion to form within cavity 101, meaning thatdroplets of one of liquids 114 and 116 are suspended in the otherliquid, and then one or more of the three voltage waveforms is applied.

The application of a bias voltage may induce or facilitate migration ofthe droplets by virtue of its effect on polar liquid 114, which in turnaffects the spatial positioning of liquid 116. Such migration may initself clear the useful optical path of the liquid lens, even if theemulsion is not completely removed. In addition, the application of thebias voltage creates a DC electric field that may cause larger dropletsto flatten and/or reduce their surface tension, This in turn encouragestheir merging or coalescence into even larger drops, which is verybeneficial to the goal of demulsification, as described below.

Independently of whether a bias voltage is applied, the application ofan oscillating actuation voltage waveform will cause the shape of themeniscus to change in a periodic manner, via normal electro-wettingprocesses, physically moving the droplets in a pumping manner.Collisions of these moving droplets with each other encouragescoalescence. Pumping can create turbulence and motivate larger dropmixing as well as release of droplets adhered to the wall of the lenscavity, which in turn can coalesce. Collisions with the cavity walls canalso encourage coalescence and migration.

Independently of whether either the bias voltage or the actuationvoltage waveforms are applied, the application of an oscillatingexcitation voltage waveform will cause the droplets to combine based onthe principles of electro-coalescence and electro-wetting. Under theforce of the electric field the suspended droplets merge together, orcoalesce, to form larger droplets. The process continues as larger andlarger droplets are formed, until ultimately there is preferably asingle predominant droplet (i.e. complete separation of the fluids).

In some embodiments, all three of the voltage types described above areapplied at substantially the same time, to achieve the goal of combiningall droplets as quickly and efficiently as possible.

In some embodiments, droplet migration can be aided by affinity betweenthe liquid in the droplets to the surfaces of the lens cavity. Thesurfaces of the lens cavity can be hydrophobic or hydrophilic, eithernaturally, by virtue of the bulk material from which the liquid lens isfabricated, or by addition of a coating, which would attract or repelthe polar fluid. Conversely, the surface can be oliophobic oroliophilic, which would attract or repel the non-polar fluidrespectively. These surface properties can be used to aid in separationof the emulsion, as the liquid lens is subject to the oscillatingvoltage for example by using an hydrophilic coating on the surface nearone electrode and an oliophilic coating on the surface near an opposingelectrode. In some embodiments, the affinity coating may be the sameelectrically insulating coating 112 overlying an electrode 106. In someembodiments, the affinity coating may be an additional coating layerdeposited over coating 112.

FIG. 3 shows a sequence of seven top-down views of an actualexperimental liquid lens in which an emulsion, clearly visible in thetop left view, is gradually separated (viewing images from left to rightin the top row and then in the bottom row) to provide a clear liquidlens in the bottom right view, according to one embodiment. The firstimage, the top left view, shows a large number of droplets, seen as darkcircles against a lighter background. The optical performance of thelens would obviously be very badly impacted by the presence of thesedroplets. However, applying a method of the present invention to thelens results in a dramatic reduction in the number of droplets,especially the smaller droplets, over time, until in the final view, nodroplets are seen at all. The time taken to achieve such clearing of anemulsion can be as short as a few seconds or a few minutes at most, asthe liquids respond to applied voltages within 10s of milliseconds.

FIGS. 4A and 4B illustrate coalescence events in two different cases. InFIG. 4A, the smaller drop is shown (above) approaching the much largerdrop, only a portion of whose surface is shown, and then (below) as itis about merge into that larger drop. In FIG. 4B, from left to right,two larger drops are shown distorting towards the smaller droppositioned between them, until the lower larger drop coalescescompletely with that smaller drop, although remaining separate from theother large drop. It should be noted that the relatively low surfacetension of larger versus smaller drops leads to the tendency of theformer to distort and merge with the latter. When two smaller drops comeinto contact, the requirement that the relatively high surface tensionof either one be overcome makes coalescence events less likely.

In the present invention, the application of a DC field may be helpfulin directly or indirectly encouraging the flattening or other distortionof droplets, making coalescence events more likely.

A typical frequency for an actuation waveform is 50 Hz, while a typicalwaveform for an excitation waveform is 100 Hz, but the values chosen forany given device may be determined according to the size of the lenscavity and the particular liquids that are used. FIG. 5 illustratessinusoidal actuation voltage waveform 502 and excitation voltagewaveform 504 for one exemplary device, where the frequency of the latteris approximately 10× the frequency of the former.

In general, the frequency or frequency band of the oscillatingexcitation waveform, their constancy or variability, and the resultingdetailed shape are tuned or tailored to stimulate electro-coalescenceaccording to the droplet size of the emulsification and the specifictwo-liquid combination used in the liquid lens. The voltage excitationsignal may be a sine, square, triangle, or saw-tooth shape or intermixedcombinations thereof. The frequency may be constant, and preferably inthe range 1 Hz to 1,000 Hz. In other cases, the frequency can vary, forexample following a swept sine function 604, as shown in FIG. 6. As thefrequency of the swept sine function increases, the liquid lens acts asa filter as in a LRC circuit, illustrated by curve 608. The liquid lensthus has a decaying amplitude response as shown by the envelope 610,which can result in a DC voltage if the frequency is specified highenough. In the case shown, where the sine function is centered on zerovolts, the final DC voltage would actually be zero.

In some embodiments, the actuation waveform applied to the liquid lenscycles from minimum voltage (Vmin) to maximum voltage (Vmax) at a periodrequired for the lens to settle near each end of its operating range.This waveform moves the lens meniscus through a desired range of focusand/or through a desired range of fluidic tilt, thereby physicallymoving the liquids in the lens cavity and encouraging drop-to-dropcontact and coalescence. The frequency of this actuation waveform istypically in the range 20 to 200 Hz, corresponding to a settling time of50 to 5 ms for focus actuation. The higher frequency excitation waveformsuperimposed on the actuation voltage waveform operates too quickly tocause meniscus movement, operating instead on the droplets of theemulsion, stimulating smaller droplets within the emulsion toagglomerate or move toward the bulk fluid interface. The frequency ofthis higher frequency signal may be in a range of 2× to 100× that of thelens actuation signal, and preferably sweep through a range offrequencies. Four options for combining or superimposing the actuationand excitation signals are shown in FIG. 7.

In some embodiments, the liquid lens can be actuated in tilt. Electrodespositioned at opposite sides of the lens cavity are actuated atdiffering voltages to cause the meniscus to tilt. The tilt actuation canbe a back and forth tilt, along the positive and negative direction ofany axis or it may oscillate around the vertical axis of symmetrythrough the lens center, like the rotation of a clock hand around the360 degree azimuth of the lens. This tilting actuation can be usedindependently or in addition to the focus actuation to stimulatecoalescence in the liquid lens.

A further aspect of the novel method proposed here for a liquid lens isthe option of incorporating a dwell time at prescribed intervals duringthe voltage excitation process. This dwell time, during which theexcitation voltage is held at a constant value, enhances the coalescenceprocess by allowing time for the droplets to build electrical charge andchange shape before being subjected to the next cycle. In one embodimenttaking advantage of this aspect, every set of 5 cycles of the actuationwaveform plus swept sine function excitation waveform is followed by adwell time of 10 ms at Vmax, before the next set of 5 cycles is applied.FIG. 8 shows another example of an electrical excitation waveform 804incorporating a dwell time 820 after each cycle of the sweep.

Advantages of the present invention over demulsification approachesdescribed in prior art include the ability to achieve rapid separationwith much lower electric fields (35 V/m has been found to work well, incomparison to the several hundred V/mm reported in the literature)avoidance of shorting issues (because liquid lenses are designed toinclude a dielectric coating covering the electrodes), and the absenceof particles or other debris that often hamper electro-coalescence inother applications (because pure liquids are used in liquid lenses).

A typical design choice for electrowetting lenses is to match thedensities of the two liquids well within the anticipated use or storagerange to eliminate any effects of gravity or accelerations on theoptical performance of the lens. However, this actually counters theability of an emulsified lens to coalesce on its own over time by thenatural settling effect of gravity. A further aspect of this inventioninvolves matching liquid densities at the higher end of the use orstorage temperature range of the product to prevent emulsification andaid the coalescing effect of the applied voltage waveforms. Elevatedtemperature reduces the viscosity thereby increasing the height of thecapillary waves that can cause break off of droplets leading to anemulsion. By matching the densities at elevated temperatures, themotivation for the fluids to emulsify is greatly reduced as predicted bythe Bond number. At lower temperatures, where the viscosity is higher,the densities can diverge a bit more because electro-coalescence is moreefficient when the densities diverge.

The methods of demulsification discussed above may be applied whenneeded, after emulsification has occurred, but may also be appliedautomatically, without regard to any specific event causingemulsification. They may, for example, be carried out as a routineprocess during powering up of the liquid lens device.

In some embodiments, the methods of demulsification discussed above maybe applied during a shock event or in anticipation of such an event. Ineither case, an acceleration sensor may be used to anticipate animpending shock force, for example a fall, and the voltage would then beapplied in response to the sensor signal. This is analogous to shockprotection used in the hard disk drive industry to prevent damage to thedisk head or magnetic platens.

A further aspect of the present invention is the possibility ofpreventing an emulsion from forming by applying a voltage determined bya control algorithm to an un-emulsified liquid lens. In this case, avoltage may be applied to the liquid lens to apply a force to theliquids that reduces their tendency to separate. For example, a specificvoltage may be used to minimize the length of the meniscus and therebylower the characteristic length parameter in the Bond number.

The present invention is also applicable to Lab-On-Chip (LOC)applications. Electro-coalescence is known to be a useful method ofachieving mixing in microfluidic channels. The principles ofelectro-coalescence and electro-wetting can be applied in combination asdisclosed above, and used to demulsify fluids or combine droplets in LOCapplications. In particular, liquid beads on a plate can be submersed inwater or other suitable fluid and then physically moved using theprinciples of electro-wetting via on-chip conductive electrodes. Themotion in coordination with the varying electric signals as describedhere can be used to enhance droplet coalescence.

Embodiments described herein provide various benefits. In particular,embodiments provide for the rapid and efficient separation of anemulsion in a liquid lens device into its component liquids, restoringthe optical clarity of the lens. Some embodiments carry out theseparation during or in anticipation of a shock event tending to causeemulsification. Some embodiments aim to prevent the formation ofemulsions.

The above-described embodiments should be considered as examples of thepresent invention, rather than as limiting the scope of the invention.Various modifications of the above-described embodiments of the presentinvention will become apparent to those skilled in the art from theforegoing description and accompanying drawings. Accordingly, thepresent invention is to be limited solely by the scope of the followingclaims.

1. A method for separating an emulsion comprising droplets of a firstliquid suspended in a second liquid within a cavity of a liquid lensdevice, the method comprising at least one of: applying a bias voltageto electrodes in the liquid lens device, causing at least one of dropletmigration, flattening of large droplets, and reduced droplet surfacetension; applying an oscillating actuation voltage waveform comprisingan actuation frequency to the electrodes, such that fluid pumping andturbulence is created within the liquid lens cavity; and applying anoscillating excitation voltage waveform comprising an excitationfrequency to the electrodes, such that the varying electric fieldcreated by the oscillating excitation voltage causes small droplets ofthe first liquid to coalesce.
 2. The method of claim 1 wherein the biasvoltage applied to the electrodes encourages migration of the firstliquid towards one side of the liquid lens cavity and migration of thesecond liquid towards an oppositely situated side of the liquid lenscavity.
 3. The method of claim 1 wherein the oscillating excitation andactuation waveforms are applied, and wherein the actuation frequency islower than the excitation frequency.
 4. The method of claim 3 whereinthe ratio of excitation frequency to actuation frequency is in the rangeof 2 to
 100. 5. The method of claim 3 wherein the actuation frequency is50 Hz and the excitation frequency is 100 Hz.
 6. The method of claim 1wherein the oscillating excitation voltage waveform comprises at leastone of a sine, square, triangle or saw-tooth waveform.
 7. The method ofclaim 1 wherein the oscillating excitation voltage waveform comprises aswept sine function.
 8. The method of claim 1 wherein the oscillatingactuation voltage waveform comprises first and second waveforms, andwherein applying the oscillating actuation voltage waveform comprisesapplying the first waveform to one electrode in the liquid lens deviceand applying the second waveform to an oppositely situated electrode inthe liquid lens device, so that a meniscus present between the first andsecond liquids experiences a tilting actuation.
 9. The method of claim 8wherein the first and second waveforms are applied in a rotationalmanner around an axis of symmetry through the liquid lens cavity suchthat the tilted meniscus rotates correspondingly around the axis, in asweeping action.
 10. The method of claim 1 wherein the oscillatingexcitation voltage waveform comprises a periodic dwell time at aconstant predetermined voltage.
 11. The method of claim 10 wherein thedwell time occurs at intervals corresponding to 5 cycles of theactuation frequency.
 12. The method of claim 1 wherein the applicationof at least one of the bias voltage, the oscillating actuation voltagewaveform and the oscillating excitation waveform occurs in response to asignal that anticipates a shock event likely to be experienced by theliquid lens device, the signal being provided by an acceleration sensor.13. A method for reducing the likelihood of emulsification occurring ina liquid lens device comprising first and second liquids, the methodcomprising: selecting the first and second liquids to have matcheddensities at a predetermined temperature that is different from thetemperature at which the liquid lens is typically used.
 14. The methodof claim 13 wherein the predetermined temperature is the maximumtemperature at which the liquid lens is intended to be used.
 15. Themethod of claim 13 wherein the predetermined temperature is the maximumtemperature at which the liquid lens is intended to be stored
 16. Amethod for reducing the likelihood of emulsification occurring inresponse to a shock event in a liquid lens device comprising first andsecond liquids, the method comprising: applying an anti-shock voltagederived by a control algorithm to electrodes in the liquid lens device,such that the length of a meniscus between the first and second liquidsis reduced; wherein the anti-shock voltage is applied independently ofany voltage waveforms used to focus the liquid lens device in normaloperation.
 17. The method of claim 16 wherein the anti-shock voltage isapplied during the shock event.
 18. The method of claim 16 wherein theanti-shock voltage is applied in response to a signal provided by asensor that anticipates a shock event likely to be experienced by theliquid lens device.
 19. The method of claim 18 wherein the sensor is anacceleration sensor.
 20. A method for separating an emulsion comprisingdroplets of a first liquid suspended in a second liquid within a cavityof a Lab-On-Chip device, the method comprising at least one of: applyinga bias voltage to electrodes in the Lab-On-Chip device, such that largedroplets of the first liquid experience flattening and reduced surfacetension; applying an oscillating actuation voltage waveform comprisingan actuation frequency to the electrodes, such that fluid pumping andturbulence is created within the cavity; and applying an oscillatingexcitation voltage waveform comprising an excitation frequency to theelectrodes, such that the varying electric field created by theoscillating voltage causes small droplets of the first liquid tocoalesce.